A Forward-Search P2MP TE LSP Inter-Domain Path Computation
draft-chen-pce-forward-search-p2mp-path-02
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draft-chen-pce-forward-search-p2mp-path-02
Internet Engineering Task Force H. Chen
Internet-Draft Huawei Technologies
Intended status: Standards Track O. Gonzalez de Dios
Expires: May 2, 2012 Telefonica I+D
October 30, 2011
A Forward-Search P2MP TE LSP Inter-Domain Path Computation
draft-chen-pce-forward-search-p2mp-path-02.txt
Abstract
This document presents a forward search procedure for computing a
path for a Point-to-MultiPoint (P2MP) Traffic Engineering (TE) Label
Switched Path (LSP) acrossing a number of domains through using
multiple Path Computation Elements (PCEs). In addition, extensions
to the Path Computation Element Communication Protocol (PCEP) for
supporting the forward search procedure are described.
Status of this Memo
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This Internet-Draft will expire on May 2, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . . 4
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Forward Search P2MP Path Computation . . . . . . . . . . . . . 5
5.1. Overview of Procedure . . . . . . . . . . . . . . . . . . 5
5.2. Description of Procedure . . . . . . . . . . . . . . . . . 6
5.3. Comparing to BRPC . . . . . . . . . . . . . . . . . . . . 8
6. Extensions to PCEP . . . . . . . . . . . . . . . . . . . . . . 8
6.1. RP Object Extension . . . . . . . . . . . . . . . . . . . 9
6.2. PCE Object . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. Candidate Node List Object . . . . . . . . . . . . . . . . 10
6.4. Node Flags Object . . . . . . . . . . . . . . . . . . . . 11
6.5. Rest Destination Nodes Object . . . . . . . . . . . . . . 11
6.6. Request Message Extension . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8.1. Request Parameter Bit Flags . . . . . . . . . . . . . . . 14
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
RFC 4105 "Requirements for Inter-Area MPLS TE" lists the requirements
for computing a shortest path for a TE LSP acrossing multiple IGP
areas; and RFC 4216 "MPLS Inter-Autonomous System (AS) TE
Requirements" describes the requirements for computing a shortest
path for a TE LSP acrossing multiple ASes. RFC 5671 "Applicability
of PCE to P2MP MPLS and GMPLS TE" examines the applicability of PCE
to path computation for P2MP TE LSPs in MPLS and GMPLS networks.
This document presents a forward search procedure to address these
requirements for computing a path for a P2MP TE LSP acrossing domains
through using multiple Path Computation Elements (PCEs).
The procedure is called "Forward Search Shortest P2MP LSP Path
Crossing Domains". The major characteristics of this procedure for
computing a path for a P2MP TE LSP from a source node to a number of
destination nodes acrossing multiple domains include the following
three ones.
1. It guarantees that the path computed from the source node to the
destination nodes is shortest.
2. It does not depend on any domain path tree or domain sequences
from the source node to the destination nodes.
3. Navigating a mesh of domains is simple and efficient.
2. Terminology
ABR: Area Border Router. Routers used to connect two IGP areas
(areas in OSPF or levels in IS-IS).
ASBR: Autonomous System Border Router. Routers used to connect
together ASes of the same or different service providers via one or
more inter-AS links.
Boundary Node (BN): a boundary node is either an ABR in the context
of inter-area Traffic Engineering or an ASBR in the context of
inter-AS Traffic Engineering.
Entry BN of domain(n): a BN connecting domain(n-1) to domain(n) along
a determined sequence of domains.
Exit BN of domain(n): a BN connecting domain(n) to domain(n+1) along
a determined sequence of domains.
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Inter-area TE LSP: A TE LSP that crosses an IGP area boundary.
Inter-AS TE LSP: A TE LSP that crosses an AS boundary.
LSP: Label Switched Path.
LSR: Label Switching Router.
PCC: Path Computation Client. Any client application requesting a
path computation to be performed by a Path Computation Element.
PCE: Path Computation Element. An entity (component, application, or
network node) that is capable of computing a network path or route
based on a network graph and applying computational constraints.
PCE(i) is a PCE with the scope of domain(i).
TED: Traffic Engineering Database.
This document uses terminologies defined in RFC5440.
3. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119.
4. Requirements
This section summarizes the requirements specific for computing a
path for a Traffic Engineering (TE) LSP acrossing multiple domains
(areas or ASes). More requirements for Inter-Area and Inter-AS MPLS
Traffic Engineering are described in RFC 4105 and RFC 4216.
A number of requirements specific for a solution to compute a path
for a TE LSP acrossing multiple domains is listed as follows:
1. The solution SHOULD provide the capability to compute a shortest
path dynamically, satisfying a set of specified constraints
across multiple IGP areas.
2. The solution MUST provide the ability to reoptimize in a
minimally disruptive manner (make before break) an inter-area TE
LSP, should a more optimal path appear in any traversed IGP area.
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3. The solution SHOULD provide mechanism(s) to compute a shortest
end-to-end path for a TE LSP acrossing multiple ASes and
satisfying a set of specified constraints dynamically.
4. Once an inter-AS TE LSP has been established, and should there be
any resource or other changes inside anyone of the ASes, the
solution MUST be able to re-optimize the LSP accordingly and non-
disruptively, either upon expiration of a configurable timer or
upon being triggered by a network event or a manual request at
the TE tunnel Head-End.
5. Forward Search P2MP Path Computation
This section gives an overview of the forward search path computation
procedure to satisfy the requirements for computing a path for a P2MP
TE LSP acrossing multiple domains described above and describes the
procedure in details.
5.1. Overview of Procedure
Simply speaking, the idea of the Forward Search P2MP inter-domain
path computation method for computing a path for an MPLS TE P2MP LSP
crossing multiple domains from a source node to a number of
destination nodes includes:
Start from the source node and the source domain.
Consider the optimal path segment from the source node to every exit
boundary node of the source domain as a special link;
Consider the optimal path segment from an entry boundary node to
every exit boundary node of a domain as a special link; and the
optimal path segment is computed as needed.
The whole topology consisting of many domains can be considered as a
special topology, which contains those special links, the normal
links in the destination domains and the inter-domain links.
Compute a shortest path in this special topology from the source node
to the multiple destination nodes using CSPF.
Forward Search P2MP inter-domain path computation method running at
any PCE just grows the result path list/tree in the same way as
normal CSPF on the special topology. When the result path list/tree
reaches all the destination nodes, the shortest path from the source
node to the destination nodes is found and a PCRep message with the
shortest path is sent to the PCE/PCC that sends the PCReq message
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eventually.
5.2. Description of Procedure
Suppose that we have the following variables:
A current PCE named as CurrentPCE which is currently computing the
path.
A number of rest destination nodes named as RestDestinationNodes,
which is the number of destination nodes to which shortest paths are
to be found. RestDestinationNodes is initially the number of all the
destination nodes of an MPLS TE P2MP LSP.
A candidate node list named as CandidateNodeList, which contains the
nodes through which the shortest path from the source node to a
destination node may be. Each node C in CandidateNodeList has the
following information:
the cost of the path from the source node to node C,
the previous hop node P and the link between P and C,
the PCE responsible for C, and
the flags for C. The flags include:
bit D indicating that node C is a Destination node if it is set;
bit S indicating that C is the Source node if it is set;
bit E indicating that C is an Exit boundary node if it is set;
bit I indicating that C is an entry boundary node if it is set; and
bit N indicating that C is a Node in a destination domain if it is
set.
The nodes in CandidateNodeList are ordered by path cost. Initially,
CandidateNodeList contains only a Source Node, with path cost 0, PCE
responsible for the source domain, and flags with S bit set.
A result path list or tree named as ResultPathTree, which contains
the shortest paths from the source node to the boundary nodes or the
nodes in the destination domains. Initially, ResultPathTree is
empty.
The Forward Search path computation method for computing a path for
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an MPLS TE P2MP LSP crossing a number of domains from a source node
to a number of destination nodes can be described as follows:
Initially, a PCC sets RestDestinationNodes to the number of all the
destination nodes of the MPLS TE P2MP LSP, ResultPathTree to empty
and CandidateNodeList to contain the source node and sends a PCE
responsible for the source domain a request with the source node,
RestDestinationNodes, CandidateNodeList and ResultPathTree.
When the PCE responsible for a domain (called current domain)
receives a request for computing the path for the MPLS TE P2MP LSP,
it checks whether the current PCE is the PCE responsible for the node
C with the minimum cost in the CandidateNodeList. If it is, then
remove C from CandidateNodeList and graft it into ResultPathTree;
otherwise, a PCReq message is sent to the PCE for node C.
Suppose that node C has Flags. The ResultPathTree is built from C in
the following steps.
If node C is a destination node (i.e., the Destination Node (D) bit
in the Flags is set), then RestDestinationNodes is decreased by one.
If RestDestinationNodes is zero (i.e., all the destinations are on
the result path tree), then the shortest path is found and a PCRep
message with the path is sent to the PCE/PCC which sends the request
to the current PCE.
If node C is in the destination domain (i.e., the Node in Destination
domain (N) bit in the Flags is set), then for every node N connected
to node C and not on ResultPathTree, it is merged into
CandidateNodeList. The cost to node N is the sum of the cost to node
C and the cost of the link between C and N. The PCE for node N is the
current PCE.
If node C is an Entry Boundary Node or Source Node (i.e., the Entry/
Incoming Boundary Node (I) bit or the Source Node (S) bit is set),
then path segments from node C to every exit boundary node of the
current domain that is not on the result path tree are computed
through using CSPF and as special links. For every node N connected
to node C through a special link (i.e., a path segment), it is merged
into CandidateNodeList. The cost to node N is the sum of the cost to
node C and the cost of the special link (i.e., path segment) between
C and N. The PCE for node N is the current PCE.
If node C is an Exit Boundary Node (i.e., the Exit Boundary Node (E)
bit is set) and there exist inter-domain links connected to it, then
for every node N connected to C and not on the result path tree, it
is merged into the candidate node list. The cost to node N is the
sum of the cost to node C and the cost of the link between C and N.
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The PCE for node N is the PCE responsible for node N.
If the CurrentPCE is the same as the PCE of the node with the minimum
cost in CandidateNodeList, then the node is removed from
CandidateNodeList, grafted to ResultPathTree, and the above steps are
repeated; otherwise, the CurrentPCE sends the PCE a request with the
source node, RestDestinationNodes, CandidateNodeList and
ResultPathTree.
5.3. Comparing to BRPC
RFC 5441 describes the Backward Recursive Path Computation (BRPC)
algorithm or procedure for computing an MPLS TE P2P LSP path from a
source node to a destination node crossing multiple domains.
Comparing to BRPC, there are a number of differences between BRPC and
the Forward-Search P2MP TE LSP Inter-Domain Path Computation. Some
of the differences are briefed below.
At first, BRPC is for computing a shortest path from a source node to
a destination node crossing multiple domains. The Forward-Search
P2MP TE LSP Inter-Domain Path Computation is for computing a shortest
path from a source node to a number of destination nodes crossing
multiple domains.
Secondly, for BRPC to compute a shortest path from a source node to a
destination node crossing multiple domains, we MUST provide a
sequence of domains from the source node to the destination node to
BRPC in advance. The Forward-Search P2MP TE LSP Inter-Domain Path
Computation does not need any sequence of domains for computing a
shortest inter-domain P2MP path.
Moreover, for a given sequence of domains domain(1), domain(2), ... ,
domain(n), BRPC searches the shortest path from domain(n), to
domain(n-1), until domain(1). Thus it is hard for BRPC to be
extended for computing a shortest path from a source node to a number
of destination nodes crossing multiple domains. The Forward-Search
P2MP TE LSP Inter-Domain Path Computation calculates a shortest path
in a special topology from the source node to the destination nodes
using CSPF.
6. Extensions to PCEP
The extensions to PCEP for Forward Search P2MP Inter-domain Path
Computation include the definition of a new flag in the RP object, a
result path list/tree and a candidate node list in a request message.
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6.1. RP Object Extension
The following flag is added into the RP Object:
The F bit is added in the flag bits field of the RP object to tell
the receiver of the message that the request/reply is for Forward
Search Path Computation.
o F (Forward search Path Computation bit - 1 bit):
0: This indicates that this is not PCReq/PCRep
for Forward Search Path Computation.
1: This indicates that this is PCReq or PCRep message
for Forward Search Path Computation.
The IANA request is referenced in Section below (Request Parameter
Bit Flags) of this document.
This F bit with the N bit defined in RFC6006 can indicate whether the
request/reply is for Forward Search Path Computation of an MPLS TE
P2MP LSP or an MPLS TE P2P LSP.
o F = 1 and N = 1: This indicates that this is a PCReq/PCRep
message for Forward Search Path Computation
of an MPLS TE P2MP LSP.
o F = 1 and N = 0: This indicates that this is a PCReq/PCRep
message for Forward Search Path Computation
of an MPLS TE P2P LSP.
6.2. PCE Object
The figure below illustrates a PCE IPv4 object body (Object-Type=2),
which comprises a PCE IPv4 address. The PCE IPv4 address object
indicates the IPv4 address of a PCE , with which a PCE session may be
established and to which a request message may be sent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PCE IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The format of the PCE object body for IPv6 (Object-Type=2) is as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCE IPv6 address (16 bytes) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.3. Candidate Node List Object
The candidate-node-list-obj object contains a list of candidate
nodes. A new PCEP object class and type are requested for it. The
format of the candidate-node-list-obj object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// <candidate-node-list> //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following is the definition of the candidate node list.
<candidate-node-list>::= <candidate-node>
[<candidate-node-list>]
<candidate-node>::= <ERO>
<candidate-attribute-list>
<candidate-attribute-list>::= [<attribute-list>]
[<PCE>]
[<Node-Flags>]
The ERO in a candidate node contain just the path segment of the last
link of the path, which is from the previous hop node of the tail end
node of the path to the tail end node. With this information, we can
graft the candidate node into the existing result path list or tree.
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Simply speaking, a candidate node has the same or similar format of a
path defined in RFC 5440, but the ERO in the candidate node just
contain the tail end node of the path and its previous hop, and the
candidate node may contain two new objects PCE and node flags.
6.4. Node Flags Object
The Node Flags object is used to indicate the characteristics of the
node in a candidate node list in a request or reply message for
Forward Search Inter-domain Path Computation. The Node Flags object
comprises a Reserved field, and a number of Flags.
The format of the Node Flags object body is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D|S|I|E|N| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
o D = 1: The node is a destination node.
o S = 1: The node is a source node.
o I = 1: The node is an entry boundary node.
o E = 1: The node is an exit boundary node.
o N = 1: The node is a node in a destination domain.
6.5. Rest Destination Nodes Object
The figure below is an illustration of an object called a number of
destinations not in path tree/list , which comprises an Object Length
field, a Class-num field, a C-type filed, and a number of
destinations not in path. As shown, the value of Object Length field
in the object may be 8, which is a length of the object in bytes; the
value of Class-num field and the value of C-type field will be
assigned by Internet Assigned Numbers Authority (IANA); and the value
of the number of destinations not in path tree field comprises a
number, which is the number of destinations that are not in the final
path computed yet.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Object Length (8) | Class-num | C-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Numbe of Destinations not in path tree |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.6. Request Message Extension
Below is the message format for a request message with the extension
of a result path list and a candidate node list:
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<PCReq Message>::= <Common Header>
[<svec-list>]
<request-list>
<request-list>::=<request>[<request-list>]
<request>::= <RP>
<END-POINTS>
[<OF>]
[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<RRO>[<BANDWIDTH>]]
[<IRO>]
[<LOAD-BALANCING>]
[<result-path-list>]
[<candidate-node-list-obj>]
[<rest-destination-nodes>]
where:
<result-path-list>::=<path>[<result-path-list>]
<path>::= <ERO><attribute-list>
<attribute-list>::=[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<IRO>]
<candidate-node-list-obj> contains a <candidate-node-list>
<candidate-node-list>::= <candidate-node>
[<candidate-node-list>]
<candidate-node>::= <ERO>
<candidate-attribute-list>
<candidate-attribute-list>::= [<attribute-list>]
[<PCE>]
[<Node-Flags>]
Figure 1: The Format for a Request Message
The definition for the result path list that may be added into a
request message is the same as that for the path list in a reply
message that is described in RFC5440.
7. Security Considerations
The mechanism described in this document does not raise any new
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security issues for the PCEP protocols.
8. IANA Considerations
This section specifies requests for IANA allocation.
8.1. Request Parameter Bit Flags
A new RP Object Flag has been defined in this document. IANA is
requested to make the following allocation from the "PCEP RP Object
Flag Field" Sub-Registry:
Bit Description Reference
18 Forward Path Computation (F-bit) This I-D
9. Acknowledgement
The author would like to thank Julien Meuric, Daniel King, Cyril
Margaria, Ramon Casellas, Olivier Dugeon and Dhruv Dhody for their
valuable comments on this draft.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC5440] Vasseur, JP. and JL. Le Roux, "Path Computation Element
(PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC6006] Zhao, Q., King, D., Verhaeghe, F., Takeda, T., Ali, Z.,
and J. Meuric, "Extensions to the Path Computation Element
Communication Protocol (PCEP) for Point-to-Multipoint
Traffic Engineering Label Switched Paths", RFC 6006,
September 2010.
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10.2. Informative References
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC5862] Yasukawa, S. and A. Farrel, "Path Computation Clients
(PCC) - Path Computation Element (PCE) Requirements for
Point-to-Multipoint MPLS-TE", RFC 5862, June 2010.
Authors' Addresses
Huaimo Chen
Huawei Technologies
Boston, MA
USA
Email: Huaimochen@huawei.com
Oscar Gonzalez de Dios
Telefonica I+D
Emilio Vargas 6, Madrid
Spain
Email: ogondio@tid.es
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